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Abstract:

The present invention provides a low-resistance magnetoresistive element
of a spin-injection write type. A crystallization promoting layer that
promotes crystallization is formed in contact with an interfacial
magnetic layer having an amorphous structure, so that crystallization is
promoted from the side of a tunnel barrier layer, and the interface
between the tunnel barrier layer and the interfacial magnetic layer is
adjusted. With this arrangement, it is possible to form a
magnetoresistive element that has a low resistance so as to obtain a
desired current value, and has a high TMR ratio.

Claims:

1. A magnetoresistive element comprising: a magnetization reference layer
having magnetization substantially perpendicular to a film plane, a
direction of the magnetization being invariable; a magnetization free
layer having magnetization substantially perpendicular to the film plane,
a direction of the magnetization being variable; and a barrier layer
provided between the magnetization reference layer and the magnetization
free layer, and having a NaCl structure, at least one of the
magnetization reference layer and the magnetization free layer including:
a first film formed in contact with the barrier layer, and comprising a
first magnetic material having a BCC structure, the first magnetic
material including at least one element selected from the group
consisting of Fe, Co, and Ni, at 50 atomic % or higher; a second film
formed in contact with the first film on the opposite side from the
barrier layer, and including at least one element selected from the group
consisting of Ta, W, Mo, Nb, Ti, Hf, and Zr; and a third film formed on
the opposite side of the second film from the first film, the third film
comprising a second magnetic material containing at least one element
selected from the group consisting of Fe, Co, and Ni, the magnetization
direction of the magnetization free layer being variable by flowing a
current between the magnetization reference layer and the magnetization
free layer via the barrier layer.

2. The element according to claim 1, wherein the second film has a film
thickness of 1 nm or less.

3. The element according to claim 1, wherein the first film has a film
thickness of 0.1 nm or more but 5 nm or less.

4. A magnetoresistive element comprising: a magnetization reference layer
having magnetization substantially perpendicular to a film plane, a
direction of the magnetization being invariable; a magnetization free
layer having magnetization substantially perpendicular to the film plane,
a direction of the magnetization being variable; and a barrier layer
provided between the magnetization reference layer and the magnetization
free layer, and having a NaCl structure, at least one of the
magnetization reference layer and the magnetization free layer including:
a first film formed in contact with the barrier layer, and comprising a
first magnetic material having a BCC structure, the first magnetic
material including at least one element selected from the group
consisting of Fe, Co, and Ni, at 50 atomic % or higher; a second film
formed in contact with the first film on the opposite side from the
barrier layer, and including at least one element selected from the group
consisting of Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Vb, and Lu; and
a third film formed on the opposite side of the second film from the
first film, the third film comprising a second magnetic material
containing at least one element selected from the group consisting of Fe,
Co, and Ni, the magnetization direction of the magnetization free layer
being variable by flowing a current between the magnetization reference
layer and the magnetization free layer via the barrier layer.

5. The element according to claim 4, wherein the element included in the
second film is Gd, and the second film has a film thickness of 0.1 nm or
more but 2 nm or less.

6. The element according to claim 4, wherein the element included in the
second film is at least one element selected from the group consisting of
Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and the second film
has a film thickness of 0.1 nm or more but 10 nm or less.

7. The element according to claim 4, wherein the first film has a film
thickness of 0.1 nm or more but 5 nm or less.

8. A magnetoresistive element comprising: a magnetization reference layer
having magnetization substantially perpendicular to a film plane, a
direction of the magnetization being invariable; a magnetization free
layer having magnetization substantially perpendicular to the film plane,
a direction of the magnetization being variable; and a barrier layer
provided between the magnetization reference layer and the magnetization
free layer, and having a NaCl structure, at least one of the
magnetization reference layer and the magnetization free layer including:
a first film formed in contact with the barrier layer, and comprising a
first magnetic material having a BCC structure, the first magnetic
material including at least one element selected from the group
consisting of Fe, Co, and Ni, at 50 atomic % or higher; a second film
formed in contact with the first film on the opposite side from the
barrier layer, and including at least one element selected from the group
consisting of Si, Ge, and Ga; and a third film formed on the opposite
side of the second film from the first film, the third film comprising a
second magnetic material containing at least one element selected from
the group consisting of Fe, Co, and Ni, the magnetization direction of
the magnetization free layer being variable by flowing a current between
the magnetization reference layer and the magnetization free layer via
the barrier layer.

9. The element according to claim 8, wherein the first film has a film
thickness of 0.1 nm or more but 5 nm or less.

10. A magnetoresistive random access memory, comprising the
magnetoresistive element according to claim 1 as a memory cell.

11. A magnetoresistive random access memory, comprising: a memory cell
that includes the magnetoresistive element according to claim 1, and a
transistor having one end series-connected to one end of the
magnetoresistive element; a first write current circuit connected to the
other end of the magnetoresistive element; and a second write current
circuit connected to the other end of the transistor, and, in cooperation
with the first write current circuit, flowing the current between the
magnetization reference layer and the magnetization free layer via the
barrier layer.

12. A magnetoresistive random access memory, comprising the
magnetoresistive element according to claim 4 as a memory cell.

13. A magnetoresistive random access memory, comprising: a memory cell
that includes the magnetoresistive element according to claim 4, and a
transistor having one end series-connected to one end of the
magnetoresistive element; a first write current circuit connected to the
other end of the magnetoresistive element; and a second write current
circuit connected to the other end of the transistor, and, in cooperation
with the first write current circuit, flowing the current between the
magnetization reference layer and the magnetization free layer via the
barrier layer.

14. A magnetoresistive random access memory, comprising the
magnetoresistive element according to claim 8 as a memory cell.

15. A magnetoresistive random access memory, comprising: a memory cell
that includes the magnetoresistive element according to claim 8, and a
transistor having one end series-connected to one end of the
magnetoresistive element; a first write current circuit connected to the
other end of the magnetoresistive element; and a second write current
circuit connected to the other end of the transistor, and, in cooperation
with the first write current circuit, flowing the current between the
magnetization reference layer and the magnetization free layer via the
barrier layer.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a divisional application of and claims the
benefit of priority under 35 U.S.C. §120 from U.S. application Ser.
No. 12/790,171, filed May 28, 2010, which is a continuation application
of U.S. Ser. No. 12/211,388, filed Sep. 16, 2008, now U.S. Pat. No.
7,768,825, which is based upon and claims the benefit of priority under
35 U.S.C. §119 from prior Japanese Application No. 2007-248251,
filed Sep. 25, 2007, the entire contents of each of which is incorporated
herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a magnetoresistive element and a
magnetoresistive random access memory including the magnetoresistive
element.

[0004] 2. Related Art

[0005] In recent years, a number of solid-state memories that record
information have been suggested on the basis of novel principles. Among
those solid-state memories, magnetoresistive random access memories
(hereinafter also referred to as MRAMs) that take advantage of tunneling
magneto resistance (hereinafter also referred to as TMR) have been known
as solid-state magnetic memories. Each MRAM includes magnetoresistive
elements (hereinafter also referred to as MR elements) that exhibit
magnetoresistive effects as the memory elements of memory cells, and the
memory cells store information in accordance with the magnetization
states of the MR elements.

[0006] Each MR element includes a magnetization free layer having a
magnetization where a magnetization direction is variable, and a
magnetization reference layer having a magnetization of which a direction
is invariable. When the magnetization direction of the magnetization free
layer is parallel to the magnetization direction of the magnetization
reference layer, the MR element is put into a low resistance state. When
the magnetization direction of the magnetization free layer is
antiparallel to the magnetization direction of the magnetization
reference layer, the MR element is put into a high resistance state. The
difference in resistance is used in storing information.

[0007] As a method of writing information on such a MR element, a
so-called current-field write method has been known. By this method, a
line is placed in the vicinity of the MR element, and the magnetization
of the magnetization free layer of the MR element is reversed by the
magnetic field generated by the current flowing through the line. When
the size of the MR element is reduced to form a small-sized MRAM, the
coercive force Hc of the magnetization free layer of the MR element
becomes larger. Therefore, in a MRAM of the current-field write type, the
current required for writing tends to be larger, since the MRAM is
small-sized. As a result, it is difficult to use a low current and
small-sized memory cells designed to have capacity larger than 256 Mbits.

[0008] As a write method designed to overcome the above problem, a write
method that utilizes spin momentum transfers (SMT) (a spin-injection
writing method or spin-transfer-torque writing method) has been suggested
(see U.S. Pat. No. 6,256,223). By the spin-injection write method, a
current is applied in a direction perpendicular to the film plane of each
of the films forming a MR element having a tunneling magnetoresistive
effect, so as to change (reverse) the magnetization state of the MR
element.

[0009] In a magnetization reversal caused by spin injection, the current
Ic required for the magnetization reversal is determined by the current
density Jc. Accordingly, as the area of the face on which the current
flows becomes smaller in a MR element, the injection current Ic required
for reversing the magnetization becomes smaller. In a case where writing
is performed with fixed current density, the current Ic becomes smaller,
as the size of the MR element becomes smaller. Accordingly, the
spin-injection writing method provides excellent scalability in
principle, compared with the current-induced magnetic field writing
method.

[0010] In a MRAM of a spin injection type, however, the current that can
be applied at the time of writing is determined by the voltage generated
at a selective transistor and the relationship in resistance between the
selective transistor and each TMR element. Therefore, it is necessary to
lower the resistance of each TMR element, or to lower the resistance of
each TMR film.

SUMMARY OF THE INVENTION

[0011] The present invention has been made in view of these circumstances,
and an object thereof is to provide a magnetoresistive element of a
low-resistance spin-injection writing type that allows the magnetization
free layer to have a magnetization reversal with a low current, and a
magnetoresistive random access memory that includes the magnetoresistive
element.

[0012] A magnetoresistive element according to a first aspect of the
present invention includes: a magnetization reference layer having
magnetization perpendicular to a film plane, a direction of the
magnetization being invariable in one direction; a magnetization free
layer having magnetization perpendicular to the film plane, a direction
of the magnetization being variable; and an intermediate layer provided
between the magnetization reference layer and the magnetization free
layer, at least one of the magnetization reference layer and the
magnetization free layer including: an interfacial magnetic layer formed
in contact with the intermediate layer, and having a crystalline phase
crystallized from an amorphous structure; and a crystallization promoting
layer formed in contact with the interfacial magnetic layer on the
opposite side from the intermediate layer, and promoting crystallization
of the interfacial magnetic layer, the magnetization direction of the
magnetization free layer being variable by flowing a current between the
magnetization reference layer and the magnetization free layer via the
intermediate layer.

[0013] A magnetoresistive element according to a second aspect of the
present invention includes: a magnetization reference layer having
magnetization parallel to a film plane, a direction of the magnetization
being invariable in one direction; a magnetization free layer having
magnetization parallel to the film plane, a direction of the
magnetization being variable; and an intermediate layer provided between
the magnetization reference layer and the magnetization free layer, at
least one of the magnetization reference layer and the magnetization free
layer including: an interfacial magnetic layer formed in contact with the
intermediate layer, and having a crystalline phase crystallized from an
amorphous structure; and a crystallization promoting layer formed in
contact with the interfacial magnetic layer on the opposite side from the
intermediate layer, and promoting crystallization of the interfacial
magnetic layer, the magnetization direction of the magnetization free
layer being variable by flowing a current between the magnetization
reference layer and the magnetization free layer via the intermediate
layer.

[0014] A magnetoresistive random access memory according to a third aspect
of the present invention includes: the magnetoresistive element according
to any one of the first and second aspects as a memory cell.

[0015] A magnetoresistive random access memory according to a fourth
aspect of the present invention includes: a memory cell that includes the
magnetoresistive element according to any one of the first and second
aspects, and a transistor having one end series-connected to one end of
the magnetoresistive element; a first write current circuit connected to
the other end of the magnetoresistive element; and a second write current
circuit connected to the other end of the transistor, and, in cooperation
with the first write current circuit, flowing the current between the
magnetization reference layer and the magnetization free layer via the
intermediate layer.

BRIEF DESCRIPTION OF THE DRAWING

[0016]FIG. 1 is a cross-sectional view of a TMR element in accordance
with a first embodiment of the present invention;

[0017]FIG. 2 is a cross-sectional view of a TMR element in accordance
with a first modification of the first embodiment;

[0018]FIG. 3 is a cross-sectional view of a TMR element in accordance
with a second modification of the first embodiment;

[0019]FIG. 4 is a cross-sectional view of a TMR element in accordance
with a fourth modification of the first embodiment;

[0020]FIG. 5 is a cross-sectional view of a TMR element in accordance
with an example 1;

[0021]FIG. 6 is a cross-sectional view of a TMR element in accordance
with an example 2;

[0022]FIG. 7 is a cross-sectional view of a TMR element in accordance
with an example 3;

[0023]FIG. 8 is a cross-sectional view of a TMR element in accordance
with an example 6;

[0024]FIG. 9 is a cross-sectional view of a TMR element in accordance
with an example 7;

[0025]FIG. 10 is a cross-sectional view of a memory cell in a MRAM in
accordance with a second embodiment; and

[0026]FIG. 11 is a circuit diagram for showing the principle components
of the MRAM of the second embodiment.

DESCRIPTION OF THE EMBODIMENTS

[0027] The following is a description of embodiments of the present
invention, with reference to the accompanying drawings.

[0028] In the following description, like components having like functions
and structures are denoted by like reference numerals, and explanation is
repeated only where necessary.

First Embodiment

[0029]FIG. 1 shows a magnetoresistive element (hereinafter also referred
to as a TMR element) in accordance with a first embodiment of the present
invention. The TMR element 1 of this embodiment includes: a magnetization
reference layer 2 that includes a magnetic film having a magnetization of
which a direction is invariable in one direction; a magnetization free
layer 6 that includes a magnetic film having a magnetization of which a
direction is variable; and an intermediate layer 4 that is provided
between the magnetization reference layer 2 and the magnetization free
layer 6.

[0030] In general, a TMR element is an element that is designed to be in
one of two steady states in accordance with the direction of the
bidirectional current flowing in a direction perpendicular to the film
plane. The two steady states are associated with "0" date and "1" data,
respectively, so that the TMR element can store binary data. This is
called the spin-injection writing type (or spin-transfer-torque writing
type), by which the magnetization is varied with the direction of the
current flowing direction, and information corresponding to the
magnetization state is stored. When there is the "0" data, the
magnetization directions of the magnetization free layer 6 and the
magnetization reference layer 2 are in a parallel state. When there is
the "1" data, the magnetization directions are in an antiparallel state.
The magnetization directions of the magnetization free layer 6 and the
magnetization reference layer 2 are substantially parallel to the film
plane, or are substantially perpendicular to the film plane. The
magnetization substantially parallel to the film plane will be
hereinafter also referred to as the in-plane magnetization, and the
magnetization substantially perpendicular to the film plane will be
hereinafter also referred to as the perpendicular magnetization.

[0031] In this embodiment, the magnetization reference layer 2 includes an
interfacial magnetic layer 2a, a crystallization promoting layer 2b, and
an assisting magnetic film 2c. The interfacial magnetic layer 2a is
provided on the opposite side of the intermediate layer 4 from the
magnetization free layer 6. The assisting magnetic film 2c is provided on
the opposite side of the interfacial magnetic layer 2a from the
intermediate layer 4. The crystallization promoting layer 2b is provided
between the interfacial magnetic layer 2a and the assisting magnetic film
2c.

[0032] Also, the magnetization free layer 6 includes an interfacial
magnetic layer 6a, a crystallization promoting layer 6b, and an assisting
magnetic film 6c. The interfacial magnetic layer 6a is provided on the
opposite side of the intermediate layer 4 from the magnetization
reference layer 2. The assisting magnetic layer 6c is provided on the
opposite side of the interfacial magnetic layer 6a from the intermediate
layer 4. The crystallization promoting layer 6b is provided between the
interfacial magnetic layer 6a and the assisting magnetic layer 6c.

(First Modification)

[0033]FIG. 2 shows a TMR element in accordance with a first modification
of this embodiment. The TMR element 1A of the first modification differs
from the TMR element 1 of the first embodiment shown in FIG. 1 in that
the magnetization free layer is a single-layer interfacial magnetic layer
without the crystallization promoting layer 6b and the assisting magnetic
layer 6c, and a crystallization promoting layer 8 is provided on the
opposite side of the magnetization free layer 6 from the intermediate
layer 4.

(Second Modification)

[0034]FIG. 3 shows a TMR element in accordance with a second modification
of this embodiment. The TMR element 1B of the second modification differs
from the TMR element 1 of the first embodiment shown in FIG. 1 in that
the magnetization reference layer 2 is a single-layer interfacial
magnetic layer without the crystallization promoting layer 2b and the
assisting magnetic layer 2c, and a crystallization promoting layer 10 is
provided on the opposite side of the magnetization reference layer 2 from
the intermediate layer 4.

(Third Modification)

[0035]FIG. 4 shows a TMR element in accordance with a third modification
of this embodiment. The TMR element 1C of the third modification differs
from the TMR element 1A of the first modification shown in FIG. 2 in that
the magnetization reference layer 2 is a single-layer interfacial
magnetic layer without the crystallization promoting layer 2b and the
assisting magnetic layer 2c, and a crystallization promoting layer 10 is
provided on the opposite side of the magnetization reference layer 2 from
the intermediate layer 4.

[0036] As described above, in each of the TMR elements in accordance with
this embodiment and its modifications, a stacked structure formed with an
intermediate layer, an interfacial magnetic layer, and a crystallization
promoting layer (a crystallization promoting layer) is provided at both
sides of the intermediate layer 4. However, the stacked structure may be
provided only on one side of the intermediate layer 4.

[0037] Further, in this embodiment, the first modification, and the second
modification, either the magnetization reference layer 2 or the
magnetization free layer 6 has an assisting magnetic layer. This
assisting magnetic layer is designed to generate anisotropy energy, when
assisting the perpendicular magnetization of either the magnetization
reference layer 2 or the magnetization free layer 6, or increasing the
resistance to thermal disturbance.

[0038] In this embodiment and its modifications, the crystallization
promoting layers 8 and 10 and the crystallization promoting layers 2b and
6b have an "effect of facilitating crystallization of an interfacial
magnetic layer". When crystallization of an interfacial magnetic layer is
started from the interface with the intermediate layer 4, the lattice
mismatching at the interface is smaller, and lower resistance and a
higher TMR ratio can be expected. As incidental effects, unnecessary
oxygen is absorbed from the interfacial magnetic layer, and
crystallization is facilitated.

[0039] In a case where a tunnel barrier layer made of an insulating oxide
material (hereinafter also referred to as the barrier layer) is used as
the intermediate layer 4, the crystallization promoting layers and the
crystallization promoting layers have the effect of absorbing excessive
oxygen from the barrier layer to make the barrier layer similar to the
stoichiometric composition and to prevent the barrier layer from being in
a peroxidative state. This interfacial magnetic layer mainly has an
amorphous structure immediately after its formation. Here, the "mainly"
means that the amorphous structure occupies 50% or more area when
observed in the film plane, or 50% or more of the volume in the film
plane is the amorphous structure. An "amorphous structure" does not have
long-range order like crystals, but has short-range order. Because of the
crystalline structure, only the first-nearest atoms are defined. The
number of the first-nearest atoms and the types of atoms can be analyzed
by a technique such as the EXAFS (Extended X-ray Adsorption Fine
Structure) technique. Also, the embodiment of the present invention
contains a polycrystalline film having mean crystal grains of 2 nm or
smaller in diameter. This is because chances are that the structure
cannot be determined whether to be a crystalline structure or an
amorphous structure. Although the interfacial magnetic layer in the
embodiment of the present invention is an amorphous structure immediately
after the film formation, the interfacial magnetic layer is
characteristically crystallized from the amorphous structure by injecting
excitation energy generated by carrying out heat treatment immediately
after the film formation, generating Joule heat a current, or performing
ion irradiation.

[0040] In this embodiment and its modifications, the intermediate layer 4
is a tunnel barrier layer (hereinafter also referred to as the barrier
layer). The barrier layer is made of an oxide having a NaCl structure.
Specific examples of the materials that can be used for the barrier layer
include CaO, MgO, SrO, BaO, and TiO, which are oxides of Be, Ca, Mg, Sr,
Ba, Ti, and the likes. Alternatively, the barrier layer may be made of a
mixed crystal material of those oxides. With the easiness of formation
and the processability of the barrier layer being taken into
consideration, MgO is practical and exhibits the highest MR ratio.

[0041] If the above described barrier layer of a NaCl structure has an
epitaxially matched interface formed on the (001) plane with respect to a
Fe1-x-yCoxNi.sub.y (0≦x≦1, 0≦y≦1,
0≦x+y≦1) magnetic film having a BCC (body-centered cubic)
structure, a high TMR ratio can be achieved. If a high TMR ratio is
achieved, the following preferred relationships should be established
between the (001) plane of the barrier layer and the (001) plane of the
magnetic film of a BCC structure:

[0042] direction of barrier layer//[110] direction of magnetic film of BCC
structure

[0043] (100) plane of barrier layer//(100) plane of magnetic film of BCC
structure

[0044] Here, the symbol "//" means "being parallel".

[0045] It is preferable that the lattice misfit at the interface is small,
so as to maintain the above orientation and direction relationships.

[0046] Further, if good lattice matching is maintained at the interface,
the bonding between the band structures of the magnetic film and the
barrier layer is good in an electronic state, and coherent electronic
tunneling occurs. Ideally, when coherent electron tunneling occurs, the
resistance of the TMR element including the magnetization reference layer
2, the tunnel barrier layer 4, and the magnetization free layer 6 becomes
lower, and a high TMR ratio can be expected. To cause coherent tunneling
in such a case, lattice matching is required at both interfaces of the
barrier layer of a NaCl structure.

[0047] In a case where the (100) plane of the barrier layer and the (110)
plane of the magnetic film of a BCC structure form an interface, the
product of resistance and area RA standardized by the area becomes 10 to
100 times higher than the RA observed in a case of an matched interface,
due to an increase in interfacial lattice misfit dislocation. Here, R
represents the resistance of the element, and A represents the area of
the element.

[0048] In a case where the barrier layer of a NaCl structure is grown
directly on the (100) plane of the above described magnetic film having a
BCC structure, it is difficult to reduce deformation of the MgO lattice,
and a misfit dislocation occurs to lower the TMR ratio. This is because
mismatching is caused at the interface due to the misfit dislocation at
the interface.

[0049] For the reasons described above, it is very difficult to grow a
barrier layer with a NaCl structure orientated to the (100) plane on the
(100) plane of a magnetic film of a BCC structure. In such a case, other
than the (100) plane, a mixed phase state in which the (111) plane is
mixed with the (100) plane appears, and crystal grains orientated to the
(100) plane and crystal grains orientated to the (111) plane exist at
random. Accordingly, the energy increases caused by the interface misfit
with the (100) plane of the magnetic film serving as the under layer can
be reduced. Thus, the misfit dislocation due to the lattice misfit at the
interface becomes larger, and the product of resistance and area RA of
the TMR element becomes higher.

[0050] The barrier layer with a NaCl structure on the under layer having
an amorphous structure, as described above, easily has crystal growth
preferentially orientated to the (100) plane. If the interfacial magnetic
layer in the embodiment of the present invention is to function as the
under layer to form a barrier layer of a TMR element, an optimum material
is an alloy formed by adding a half-metal element such as B, P, S, or C,
or N (nitrogen), or a semiconductor element such as Si, Ge, or Ga to a
Fe1-x-yCoxNi.sub.y (0≦x≦1,0≦y≦1,
0≦x+y≦1) alloy inherently having a BCC structure. Each of
those materials is crystallized through an excitation process such as
heat treatment, and a BCC structure phase is then precipitated.

[0051] The optimum thickness of the interfacial magnetic layer is in the
range of approximately 0.1 nm to 5 nm. If the thickness of the
interfacial magnetic layer is smaller than 0.1 nm, a high TMR ratio and a
low RA cannot be achieved. If the thickness exceeds 5 nm, the interfacial
magnetic layer is much larger than the characteristic length with which a
spin torque can be applied, and the magnetization free layer cannot have
a magnetization reversal through spin injection. Therefore, the thickness
of the interfacial magnetic layer is optimized so as to restrict the film
thickness of the magnetization free layer to 5 nm or less.

[0052] The interfacial magnetic layer of the embodiment of the present
invention should preferably contain at least one element selected from
the group consisting of Fe, Co, and Ni at 50 atomic % or more. This is
because, if the interfacial magnetic layer contains the element at less
than 50 atomic %, magnetization might disappears. In such a case, there
is a high probability that the polarizability of the interfacial magnetic
layer also becomes lower and disappears. Even if the resistance can be
lowered, a MR ratio cannot be measured.

[0053] To crystallize the above-mentioned amorphous phase, it is necessary
to carry out heat treatment. When a BCC structure is precipitated through
crystallization, it is necessary to prepare the origin of
crystallization.

[0054] In this embodiment and its modifications, the interfacial magnetic
layer having an amorphous structure stabilizes the entire energy through
crystallization originated from the interface with the intermediate layer
4. More specifically, to achieve a high TMR ratio with a low current, it
is necessary to facilitate crystallization of the interfacial magnetic
layer having an amorphous structure, starting from the interface side of
the tunnel barrier layer 4. In the case where crystallization starts from
the side of the tunnel barrier layer 4, it is considered that the
crystallization progresses by minimizing the misfit energy at the
interface with the tunnel barrier layer while an appropriate amount of an
additional material such as B, P, S, C, or N is contained. As a result, a
very small amount of B, P, S, C, or N remains in the phase of the
magnetic film of a BCC structure after recrystallization. Thus, the
misfit dislocations at the interface can be restricted to a low amount.

[0055] In the above described interfacial magnetic layer, crystallization
from an amorphous phase occurs first at the interface with lower
interface energy. In this case, the crystallization structure and
orientation after the crystallization are determined so that the
interface energy of the interface at which the crystallization starts is
reduced.

[0056] In this embodiment and its modifications, the interfacial magnetic
layer having an amorphous structure is in contact with the barrier layer
4 of a NaCl structure on its (001) plane. Therefore, crystallization
should ideally start at the interface on the side of the barrier layer 4
of a NaCl structure. In this case, the interfacial magnetic layer
inevitably forms an interface epitaxially matching with the barrier layer
4 of a NaCl structure from an amorphous structure, and a BCC structure
phase grows from this interface, orientated to the (001) plane. Here, the
crystallization progresses while the following crystalline direction
relationships (also described above) are maintained:

[0057] direction of barrier layer//[110] direction of magnetic film of BCC
structure

[0058] (100) plane of barrier layer//(100) plane of magnetic film of BCC
structure

[0059] The inventors discovered what kind of layer should be brought into
contact with the interface on the opposite side of the interfacial
magnetic layer (crystallizing from an amorphous phase) from the barrier
layer, so that the magnetic film having an amorphous structure is
crystallized from the barrier layer side as described above. In other
words, to facilitate crystallization from the barrier layer side, a layer
made of a material that is crystallized at a lower speed is formed on the
side of the other interface.

[0060] Here, the entire magnetic layer having an amorphous structure does
not need to be crystallized, but the interface with the intermediate
layer should be crystallized. The spin-injection magnetization reversal
current might become lower in a case where only the interface with the
intermediate layer (the barrier layer) and its neighboring area are
crystallized.

[0061] More specifically, it is preferable to employ an element that forms
a eutectic state with Fe, Co, and Ni, instead of a total solid-soluble
state. It is more preferable to employ an element having a higher melting
point than Fe, Co, and Ni. The melting points of Fe, Co, and Ni are
1540° C., 1490° C., and 1450° C., respectively.

[0062] As for the crystalline structure of the magnetic film, it is
preferable to use an element having a BCC structure or a hexagonal closed
pack (HCP) structure, other than a face-closed cubic (FCC) structure.
Alternatively, it is preferable to use a covalent bonding element.

[0063] Next, materials that can be used for the crystallization promoting
layers (or the crystallization promoting layers) in this embodiment and
its modifications are described.

[0064] In each TMR element of this embodiment and its modifications, the
crystallization promoting layer (or the crystallization promoting layer)
is made of a rare earth element selected from the group consisting of Ce,
Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu (hereinafter also
referred to as the element A). Those rare earth elements have almost no
solid solubility limits, when combined with Fe, Co, or Ni. Each magnetic
film in the magnetization free layers and the magnetization reference
layers of this embodiment and the first modification is made of an alloy
containing an element selected from the group consisting of Fe, Co, and
Ni, and is hardly solid soluble in a rare earth metal.

[0065] At the time of film formation by a sputtering technique or the
like, the crystallization promoting layer (the crystallization promoting
layer) causes mixing with the interfacial magnetic layer in an amorphous
state at the interface, and partially forms an amorphous phase of Fe, Co,
or Ni and a rare earth metal. The amorphous phase of Fe, Ni, or Co and a
rare earth element has a high crystallization temperature. Accordingly,
crystallization of the interfacial magnetic layer from an amorphous phase
easily starts at the interface between the interfacial magnetic layer and
the barrier layer.

[0066] Among rare earth elements, Gd is a ferromagnetic material having
spontaneous magnetization. Accordingly, the exchange coupling between the
interfacial magnetic layers 2a and 6a and the assisting magnetic layers
2c and 6c is not cut off by the addition of the crystallization promoting
layers 2b and 6b. Also, in a case where Gd is employed, there are no
upper limits set on the thicknesses of the crystallization promoting
layers 2b and 6b, as long as they are in-plane magnetization films. The
lower limit of the film thicknesses is 0.1 nm. If the crystallization
promoting layers 2b and 6b are thinner than 0.1 nm, the insertion effect
cannot be achieved. In a case where the magnetization free layer and the
magnetization reference layer including the crystallization promoting
layers have perpendicular magnetization, the thickness of the Gd layer
should preferably be 2 nm or less, so as to maintain the perpendicular
properties of the layer containing Gd.

[0067] Meanwhile, elements such as Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm,
Yb, and Lu do not have magnetism as single metals. However, each of those
elements is alloyed with an element selected from the group consisting of
Fe, Co, and Ni, and forms an amorphous structure or an intermetallic
compound. In this manner, those elements come to have magnetism generated
from the orbital moment. Likewise, Gd is alloyed with an element selected
from the group consisting of Fe, Co, and Ni, and forms an amorphous
structure or an intermetallic compound. In this manner, Gd comes to have
magnetism generated from the orbital moment.

[0068] In a case where a high-energy film formation process involving a
sputtering technique or the like is carried out on a single metal made of
a rare metal element selected from the group consisting of Ce, Pr, Nd,
Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Lu, an amorphous phase is formed when
mixing with the magnetic film serving as the under layer is caused.
Likewise, a mixing layer is formed on the rare-earth single-metal layer,
and an amorphous phase is formed. Accordingly, the magnetic coupling or
exchange coupling between the interfacial magnetic layers 2a and 6a and
the assisting magnetic layers 2c and 6c is not cut off. In this case, the
film thickness of each crystallization promoting layer should be 1 nm or
less. This is because the mixing is caused approximately 0.5 nm above or
below the crystallization promoting layer.

[0069] The crystallization promoting layer (the crystallization promoting
layer) having magnetism and being exchange-coupled to the interfacial
magnetic layer or the assisting magnetic layer is essential in a case
where the magnetization reference layer and the magnetization free layer
have perpendicular magnetization.

[0070] For example, in many cases, the magnetization reference layer 2 and
the assisting magnetic layers 2c and 6c used in the magnetization free
layer 6 have perpendicular magnetization, and the interfacial magnetic
layers 2a and 6a have in-plane magnetic films, as shown in FIG. 1. To
cause the interfacial magnetic layers 2a and 6a to have perpendicular
magnetization in this case, it is necessary to maintain the exchange
coupling between the crystallization promoting layers 2b and 6b and the
interfacial magnetic layers 2a and 6a, and the exchange coupling between
the crystallization promoting layers 2b and 6b and the crystallization
promoting layers 2c and 6c.

[0071] When a rare earth element is alloyed with Co, Ni, or Fe
(hereinafter also referred to as the element X), an amorphous structure
is formed. The definition of an amorphous structure has already been
described. An alloy of an amorphous structure formed with a rare earth
element and the element X (Fe, Co, or Ni) can have perpendicular
magnetization. An amorphous alloy formed with a rare earth element and
the element X is a ferrimagnetic body. Accordingly, the amorphous alloy
has the compensation point composition with which the net saturation
magnetization Ms is zero. The compensation point composition is indicated
by the atomic percentage (atomic %) of the rare earth element. If the
amount of the rare earth element exceeds the compensation point
composition, the saturation magnetization Ms takes a negative value. In
other words, the field applying direction and the magnetization direction
become opposite to each other. Since the saturation magnetization Ms
becomes smaller when the composition is close to the compensation point,
the effective magnetic crystalline anisotropy (Ku-effect) becomes
larger, and stable perpendicular magnetization can be readily achieved.
Accordingly, the above-described XA amorphous alloy is the optimum
material for the crystallization promoting layer (the crystallization
promoting layer) in a case of a MR element having perpendicular
magnetization. Examples of amorphous structure alloys that have
perpendicular magnetization and are formed with rare earth elements and
the element A include a TbCoFe alloy, a GdCoFe alloy, and a TbGdCoFe
alloy. It is also possible to add Ho or Dy to those alloys.

[0072] The crystallization promoting layer made of a rare earth element A
selected from the group consisting of Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho,
Er, Tm, Yb, and Lu should preferably remain an amorphous structure even
after the interfacial magnetic layer is crystallized, except for the case
where the element A is Gd. The film thickness of the crystallization
promoting layer (the crystallization promoting layer) made of an
amorphous alloy formed with a rare earth element and the element X is not
limited, and should preferably be in the range of 0.1 nm to 10 nm.

[0073] In each TMR element of this embodiment and its modifications, the
crystallization promoting layer (the crystallization promoting layer) may
be made of an element selected from the group consisting of Mg, Ca, Sc,
Ti, Sr, Y, Zr, Nb, Mo, Ba, La, Hf, Ta, and W (hereinafter referred to as
the element B). Each of those elements has a BCC structure or a HCP
structure. In a case where the element B is an element selected from the
group consisting of Mg, Ca, Sc, Ti, Zr, Y, and Sr, a HCP structure is
formed. In a case where the element B is an element selected from the
group consisting of Ta, W, Nb, Mo, Ba, Hf, and La, a BCC structure is
formed.

[0074] The above-described elements have almost no solid solubility limits
with respect to Fe, Co, and Ni (hereinafter also referred to as the
element X), and each of the elements can form an intermetallic compound.
Each of those elements easily forms an amorphous structure phase, when
alloyed. As will be described later in detail, those elements exhibit
almost no solid solubility limits with a noble metal element (hereinafter
referred to as the element Y) such as Pt, Pd, Au, Ag, Ru, Rh, Ir, or Os,
or alloys.

[0075] The above-mentioned elements, Mg, Ca, Sc, Ti, Sr, Y, Zr, Nb, Mo,
Ba, La, Hf, Ta, and W, are nonmagnetic elements. Therefore, the thickness
of the crystallization promoting layer is restricted to 1 nm or less. If
the thickness exceeds 1 nm, the exchange coupling between the interfacial
magnetic layers 2a and 6a and the assisting magnetic layers 2c and 6c is
cut off. The crystallization promoting layer (the crystallization
promoting layers) that has a thickness of 1 nm or less and is made of an
element (hereinafter referred to as the element B) selected from the
group consisting of Mg, Ca, Sc, Ti, Sr, Y, Zr, Nb, Mo, Ba, La, Hf, Ta,
and W causes mixing with the layer serving as the under layer, and forms
an amorphous structure. Furthermore, the element B has a great effect of
attracting oxygen atoms contained in the interfacial magnetic layers,
because the element B has higher electronegativity than the element X
such as Fe, Co, or Ni, and easily attracts oxygen. In each interfacial
magnetic layer in this case, more oxygen is observed at locations closer
to the crystallization promoting layer (the crystallization promoting
layer). Accordingly, the oxygen concentration on the interface side of
the barrier layer is lower, and crystallization from an amorphous
structure is facilitated.

[0076] The crystallization promoting layer (the crystallization promoting
layer) made of the element B may be crystallized to form a BCC structure
or a HCP structure. In the case of a BCC structure, the orientation to
the (100) plane is observed. In the case of a HCP structure, orientation
does not matter.

[0077] In each TMR element of this embodiment and its modifications, the
crystallization promoting layer may be made of an element (hereinafter
also referred to as the element C) selected from the group consisting of
Si, Ge, and Ga. Those elements are semiconductor elements having covalent
bonding. The above-described elements have almost no solid solubility
limits with respect to Fe, Ci, and Ni, and easily form amorphous
structures when alloyed. As will be described later in detail, those
elements have almost no solid solubility limits with respect to a noble
metal element (the element Y) such as Pt, Pd, Au, Ag, Ru, Rh, Ir, or Os,
or an alloy. Therefore, each assisting magnetic layer should preferably
be an alloy or a stacked structure containing at least one element
selected from the group consisting of Fe, Co, and Ni, and at least one
element selected from the group consisting of Ru, Rh, Pd, Ag, Os, Ir, Pt,
and Au.

[0078] The assisting magnetic layers used in the embodiment of the present
invention are now described. Each of those assisting magnetic layers has
the function of assisting and reinforcing the perpendicular magnetization
characteristics of the magnetization reference layer or the magnetization
free layer, and also has the function of improving the heat disturbance
resistance of the magnetization reference layer or the magnetization free
layer. These assisting magnetic layers are designed to provide magnetic
crystalline anisotropy energy. Accordingly, in a case where the
interfacial magnetic layer has sufficient perpendicular magnetization
characteristics and heat disturbance resistance, it is not necessary to
employ the assisting magnetic layers. The thickness of the assisting
magnetic layer used in the magnetization free layer should be 5 nm or
smaller, so that spin-injection magnetization reversals can be caused. A
thickness of 5 nm or more is much larger than the characteristic length
with which a spin torque is validly applied, and the magnetization free
layer cannot have a magnetization reversal through spin injection if the
assisting magnetic layer is thicker than 5 nm. The assisting magnetic
layer used in the magnetization reference layer should have such a
thickness as not to have a reversal when the magnetization free layer has
a magnetization reversal. Therefore, the following relationship should be
established:

Ms-freetfree<Ms-referencetreference

[0079] where Ms-free, Ms-reference, tfree, and
treference represent the saturation magnetization of the
magnetization free layer, the saturation magnetization of the
magnetization reference layer, the film thickness of the magnetization
free layer, and the film thickness of the magnetization reference layer,
respectively.

[0080] The assisting magnetic layers having perpendicular magnetization
are now described. Here, "perpendicular magnetization" and "magnetization
substantially perpendicular to the film plane" are defined as the state
in which the ratio (Mr/Ms) between the residual magnetization Mr and the
saturation magnetization Ms when there is not a magnetic field is 0.5 or
higher in the magnetization-field (M-H) curve obtained by carrying out
VSM (vibration sample magnetization) measurement. The characteristic
length with which a spin torque is validly applied is approximately 1.0
nm. Examples of the materials that exhibit perpendicular magnetization
include a CoPt alloy, a CoCrPt alloy, and a CoCrPtTa alloy that have
hexagonal closed pack (HCP) structures or face-centered cubic (FCC)
structures. To exhibit magnetization perpendicular to the film plane, the
material needs to be orientated to the (001) plane in a HCP structure,
and needs to be orientated to the (111) plane in a FCC structure. A phase
transition layer having a CsCl ordered structure phase tends to be
orientated to the (110) plane.

[0081] The examples of the materials that exhibit perpendicular
magnetization also include a RE-TM alloy that is formed with a rare earth
metal (hereinafter also referred to as a RE) and an element selected from
the group consisting of Co, Fe, and Ni (hereinafter also referred to as
the TM element), and has an amorphous structure. The net saturation
magnetization of the RE-TM alloy can be controlled to switch from a
negative value to a positive value by adjusting the amount of the RE
element. The point where the net saturation magnetization Ms-net becomes
zero is called the compensation point, and the composition observed at
that point is called the compensation point composition. In the
compensation point composition, the proportion of the RE element falls in
the range of 25 atomic % to 50 atomic %.

[0082] The examples of the materials that exhibit perpendicular
magnetization also include an artificial-lattice perpendicular
magnetization film formed with multilayer stacked layers: a magnetic
layer containing an element selected from the group consisting of Co, Fe,
and Ni; and a nonmagnetic metal layer containing Pd, Pt, Au, Rh, Ir, Os,
Ru, Ag, or Cu. The material of the magnetic layer may be a
Co100-x-yFexNi.sub.y alloy film (0≦x≦100,
0≦y≦100). It is also possible to employ a CoFeNiB amorphous
alloy having B added to the above CoFeNi alloy at 10 to 25 atomic %. The
optimum film thickness of the magnetic layer is in the range of 0.1 nm to
1 nm. The optimum thickness of the nonmagnetic layer is in the range of
0.1 nm to 3 nm. The crystalline structure of the artificial lattice film
may be a HCP structure, a FCC structure, or a BCC structure. In the case
of a FCC structure, the artificial lattice film is partially orientated
to the (111) plane. In the case of a BCC structure, the artificial
lattice film is partially orientated to the (110) plane. In the case of a
HCP structure, the artificial lattice film is partially orientated to the
(001) plane. The orientation can be observed through X-ray diffraction or
electron beam diffraction.

[0083] The examples of the materials that exhibit perpendicular
magnetization also include a FCT ferromagnetic alloy that has a L10
ordered structure and is formed with at least one element selected from
the group consisting of Fe and Co (hereinafter referred to as the element
A), and at least one element selected from the group consisting of Pt and
Pd (hereinafter referred to as the element B). Typical examples of
L10 ordered structure ferromagnetic alloys include an L10-FePt
alloy, an L10-FePd alloy, and an L10-CoPt alloy. It is also
possible to employ an L10-FeCoPtPd alloy that is an alloy formed
with the above alloys. To form such a L10 ordered structure, "x"
needs to be in the range of 30 atomic % to 70 atomic %, where the
relative proportions of the element A and the element B are expressed as
A100-xBx. Part of the element A can be replaced with Ni or Cu.
Part of the element B can be replaced with Au, Ag, Ru, Rh, Ir, Os, or a
rare earth element (such as Nd, Sm, Gd, or Tb). In this manner, the
saturation magnetization Ms and the magnetic crystalline anisotropy
energy (uniaxial magnetic anisotropy energy) Ku of the magnetization
free layer having perpendicular magnetization can be adjusted and
optimized.

[0084] The above-described ferromagnetic AB alloy having a L10
ordered structure is a face-centered tetragonal (FCT) structure. By
regulating the structure, large magnetic crystalline anisotropy energy of
approximately 1×107 erg/cc can be obtained in the [001]
direction. In other words, excellent perpendicular magnetization
characteristics can be achieved through preferential orientation toward
the (001) plane. The saturation magnetization is approximately in the
range of 600 emu/cm3 to 1200 emu/cm3. In a case where an
element is added to the alloy by replacing a component with the element A
or the element B, the saturation magnetization and the magnetic
crystalline anisotropy energy become smaller. On the (001) plane of the
ferromagnetic AB alloy having the above described L10 ordered
structure, a BCC structure alloy containing Fe, Cr, V, or the like as a
principal component easily grows, preferentially orientated to the (001)
plane.

[0085] The preferential orientation of a FCT-FePt alloy to the (001) plane
can be observed as a (002) peak in the neighborhood of the point where 20
is 45 to 50 degrees by performing a θ-2θ scan with X-ray
diffraction. To improve the perpendicular magnetization characteristics,
the half width of the rocking curve of the (002) diffraction peak needs
to be 10 degrees or less, and, more preferably, 5 degrees or less.

[0086] The existence of a L10 ordered structure phase and the
preferential orientation to the (001) plane can be observed as a (001)
diffraction peak in the neighborhood of the point where 2θ is 20 to
25 degrees by performing a θ-2θ scan with X-ray diffraction.

[0087] Those diffraction images that are derived from the (001) plane and
the (002) plane can be observed through electron beam diffraction or the
like.

EXAMPLES

[0088] Next, specific stacked structures of TMR elements are described as
examples in accordance with the present invention in detail.

Example 1

[0089]FIG. 5 shows a TMR element of a coercitivity differential type as
Example 1 in accordance with the present invention. The TMR element of
Example 1 is of a bottom-reference type. More specifically, a
magnetization reference layer 2 is formed on a under layer 12, an
intermediate layer 4 is formed on the magnetization reference layer 2, a
magnetization free layer 6 is formed on the intermediate layer 4, and a
cap layer 14 is formed on the magnetization free layer 6. The
magnetization reference layer 2 is a stacked structure that includes an
assisting magnetic layer 2c formed on the under layer 12, a
crystallization promoting layer 2b formed on the assisting magnetic layer
2c, and an interfacial magnetic layer 2a formed on the crystallization
promoting layer 2b. In this example, the magnetic reference layer 2 and
the magnetization free layer 6 may both have magnetization perpendicular
to the film plane, or may both have magnetization parallel to the film
plane.

Example 2

[0090]FIG. 6 shows a TMR element of a coercitivity differential type as
Example 2 in accordance with the present invention. The TMR element of
Example 2 is of a bottom-reference type, and is the same as the TMR
element of Example 1 shown in FIG. 5, except that a crystallization
promoting layer 8 is provided between the magnetization free layer 6 and
the cap layer 14. In this example, the interfacial magnetic layer serving
as the magnetization free layer 6 may not be made of a material that is
crystallized from an amorphous structure, but may be made of a magnetic
material that is crystallized in the first place. In this case, the
crystallization promoting layer 8 plays a role of an excess oxygen
absorbing layer to adjust the stoichiometric composition of the barrier
layer (the intermediate layer) 4. In this example, the magnetic reference
layer 2 and the magnetization free layer 6 may both have magnetization
perpendicular to the film plane, or may both have magnetization parallel
to the film plane.

Example 3

[0091]FIG. 7 shows a TMR element of a coercitivity differential type as
Example 3 in accordance with the present invention. The TMR element of
Example 3 is of a top-reference type. More specifically, a magnetization
free layer 6 is formed on a under layer 12, an intermediate layer 4 is
formed on the magnetization free layer 6, a magnetization reference layer
2 is formed on the intermediate layer 4, and a cap layer 14 is formed on
the magnetization reference layer 2. The magnetization free layer 6 is a
stacked structure that includes an assisting magnetic layer 6c formed on
the under layer 12, a crystallization promoting layer 6b formed on the
assisting magnetic layer 6c, and an interfacial magnetic layer 6a formed
on the crystallization promoting layer 6b. The magnetization reference
layer 2 is a stacked structure that includes an interfacial magnetic
layer 2a formed on the intermediate layer 4, a crystallization promoting
layer 2b formed on the interfacial magnetic layer 2a, and an assisting
magnetic layer 2c formed on the crystallization promoting layer 2b. In
this example, the magnetic reference layer 2 and the magnetization free
layer 6 may both have magnetization perpendicular to the film plane, or
may both have magnetization parallel to the film plane.

[0092] Specific example structures of the respective TMR elements of
Examples 1 to 3 are shown below. The numeric values shown in the brackets
indicate film thicknesses.

[0096] The above described specific example structures are TMR elements
having perpendicular magnetization. In the case of a TMR element that has
the perpendicular magnetization shown in each specific example structure
of Examples 1 and 2, a FePt film having a L10 ordered structure is
employed as the interfacial magnetic layer 6a of the magnetization free
layer 6. A CoFeB alloy is employed as the interfacial magnetic layer 2a
of the magnetization reference layer 2, a FePt alloy is employed as the
assisting magnetic layer 2c, and a Ta film is employed as the
crystallization promoting layer 2b.

[0097] In the TMR element having the specific example structure of Example
3, a CoFeB film is employed as the interfacial magnetic layer 6a of the
magnetization free layer 6, a FePt film having a L10 ordered
structure is employed as the assisting magnetic layer 6c, and a Ta film
is employed as the crystallization promoting layer 6b. A Fe film is
employed as the interfacial magnetic layer 2a of the magnetization
reference layer 2, a FePt film having a L10 ordered structure is
employed as the assisting magnetic layer 2c, and a Mg film is employed as
the crystallization promoting layer 2b.

[0098] Here, examples of L10 alloy layers that can be used in the
magnetization free layer include not only FePt alloys but also
ferromagnetic alloys each formed with an element X that is at least one
element selected from the group consisting of Fe and Co, and an element Y
that is at least one element selected from the group consisting of Pt and
Pd. Typical examples are a FePt alloy of a L10 ordered structure, a
FePd alloy of a L10 ordered structure, and a CoPt alloy of a
L10 ordered structure. To form a L10 ordered structure, the
relative proportions of the element X and the element Y should preferably
indicate that the relative proportion of the element X is in the range of
40 atomic % to 60 atomic %. Part of a magnetization free layer made of a
XY alloy having the above described L10 ordered structure may be
replaced with Ni, Cu, Zn, or the like. In this manner, the saturation
magnetization Ms can be made lower. In a case where part of the
magnetization free layer is replaced with Cu, Zn, or the like, the
ordering temperature can be made lower.

[0099] Also, part of a magnetization free layer made of a XY alloy having
the above described L10 ordered structure may be replaced with Cu,
Au, Ag, Ru, Rh, Ir, Os, or a rare earth element (Nd, Sm, Gd, Tb, or the
like).

[0100] The above-described ferromagnetic XY alloy having the L10
ordered structure is a FCT structure. By regulating the structure, large
magnetic crystalline anisotropy energy of approximately 1×107
erg/cm3 can be obtained in the [001] direction. In other words,
excellent perpendicular magnetization characteristics can be achieved
through preferential orientation toward the (001) plane. The saturation
magnetization is approximately in the range of 600 emu/cm3 to 1100
emu/cm3. In a case where one of the above mentioned elements is
added, saturation magnetization can be made lower through optimization,
while effective magnetic crystalline anisotropy is maintained.

[0101] In the TMR elements of Examples 1 and 2 shown in FIGS. 5 and 6, the
noble metal such as Pt or Pd might diffuse into the interfacial magnetic
layer 2a having an amorphous structure, if the interfacial magnetic layer
2a is formed on the assisting magnetic layer 2c of the magnetization
reference layer 2. As a result, crystallization of the interfacial
magnetic layer 2a from an amorphous structure phase might be hindered.
Therefore, the crystallization promoting layer 2b is inserted between the
interfacial magnetic layer film 2a and the assisting magnetic layer 2c,
so that the noble metal such as Pt or Pd can be prevented from diffusing
into the interfacial magnetic layer 2a.

[0102] The crystallization promoting layer 2b serves to prevent diffusion.
In view of this, if the magnetization reference layer 2 has the assisting
magnetic layer 2c of hard magnetism at the bottom as shown in FIGS. 5 and
6, it is preferable to insert a crystallization promoting layer 2b that
is not made of an element that is not solid-soluble in noble metals such
as Pt and Pd. Other than Ta, the crystallization promoting layer 2b may
be made of Mg, Ca, Sc, Ti, Sr, Y, Zr, Nb, Mo, Ba, La, Hf, W, or the like.

[0103] The material CrFeB of the interfacial magnetic layer 2a tends to
have in-plane magnetization. Therefore, magnetic exchange coupling being
disrupted by the insertion of the crystallization promoting layer 2b is
not preferable to maintain the perpendicular magnetization
characteristics of the interfacial magnetic layer 2a. In view of this, it
is preferable to insert a rare earth element such as Ce, Pr, Nd, Sm, Eu,
Tb, Dy, Ho, Er, Tm, Yb, or Lu. This is because magnetism can be generated
by the mixing at the time of film formation. Especially, Gd is a
ferromagnetic material, even though it is a single element. To maintain
the perpendicular magnetization, it is preferable that the film thickness
of the above described crystallization promoting layer is 1 nm or
smaller. In the case of Gd, the film thickness should preferably be 2 nm
or less. However, to maintain the perpendicular magnetization of the
interfacial magnetic layer of the magnetization reference layer, the
product of the saturation magnetization Ms and the film thickness t
should preferably be 4.0 [nmT (nanometertesla)] or less.

Examples 4 and 5

[0104] Typical example of stacked structures that are TMR elements of
bottom-reference types in which Gd is employed for the crystallization
promoting layer s are now shown as Examples 4 and 5.

[0106] In a TMR element of the bottom-reference type such as the TMR
elements of Examples 1 and 2, an alloy film formed with a rare earth
element and an element X that is at least one element selected from the
group consisting of Fe and Co may be employed as the crystallization
promoting layer . An alloy film formed with a rare earth element and the
element X has perpendicular magnetization. Example 5 is a stacked
structure in a TMR element of a typical bottom-reference type.

[0108] Although the film thickness of the above CoFeTb film 2b is 5 nm, it
may be in the range of 0.1 nm to 10 nm by optimizing the film formation
process. Here, the relative proportion of Tb in the CoFeTb film is 50 vol
% or less in volume percent. The relative proportion of the rare earth
element should preferably be 50 vol % or less. If a larger amount than
that is added, the exchange coupling between the interfacial magnetic
layer 2a and the assisting magnetic layer 2c becomes weaker, and the
perpendicular magnetization might not be maintained.

Example 6

[0109]FIG. 8 shows a stacked structure of a TMR element as Example 6,
which is a structure of a bottom-reference type in which the
magnetization of the magnetization reference layer 2 is fixed by an
antiferromagnetic layer 7. The TMR element of this example shown in FIG.
8 is the same as the TMR element of Example 1 shown in FIG. 5, except
that the assisting magnetic layer 2c of the magnetization reference layer
2 is replaced with a magnetization pinned film 2d having magnetization
fixed by the antiferromagnetic layer 7. The materials used in this
example are those mentioned in Examples 1 to 5. The magnetization
directions of the magnetization pinned film 2d and the interfacial
magnetic layer 2a may be perpendicular to the film plane or may be
parallel to the film plane.

Example 7

[0110]FIG. 9 is a cross-sectional view of a TMR element as Example 7 in
accordance with the present invention. The TMR element of this example is
the same as the TMR element of Example 6, except that the magnetization
pinned film 2d that is a single-layer magnetic film is replaced with a
magnetization pinned layer 3 having a synthetic structure. More
specifically, the magnetization pinned layer 3 is a stacked structure
having a nonmagnetic film 3b provided between a magnetic film (an
interfacial magnetic layer) 3a and a magnetic film 3c. The magnetic film
3a and the magnetic film 3c are antiferromagnetically coupled to each
other via the nonmagnetic film 3b. The magnetization of the magnetization
pinned layer 3 is fixed by the antiferromagnetic layer 7. The
magnetization directions of the magnetization pinned layer 3 and the
interfacial magnetic layer 2a may be perpendicular to the film plane or
may be parallel to the film plane.

[0111] In practice, the antiferromagnetic layer 7 may be a FeMn alloy
layer, a PtMn alloy layer, an IrMn alloy layer, a NiMn alloy layer, a
PdMn alloy layer, a RhMn alloy layer, a PtCr alloy layer, a PtCrMn alloy
layer, or the like. The optimum film thickness is in the range of 5 nm to
20 nm.

[0112] In the synthetic structure, the nonmagnetic film 3 is inserted
between the interfacial magnetic layer 3a and the magnetic film 3c. The
nonmagnetic film 3b may be made of Ru, Os, or Ir, and its optimum film
thickness is in the range of 0.5 nm to 3 nm. The synthetic structure
utilizes interlayer coupling, and a film thickness with which the
antiferromagnetic coupling becomes the strongest is used. In the
synthetic structure, the magnetization directions of the interfacial
magnetic layer 3a and the magnetic film 3c are antiparallel to each
other.

[0113] In Example 7 shown in FIG. 9, the crystallization promoting layer
2b is inserted to an interfacial magnetic layer, and divides the
interfacial magnetic layer into the interfacial magnetic layer 2a and the
interfacial magnetic layer 3a. The film thickness of the interfacial
magnetic layer 2a that is located near the barrier layer 4 needs to be 1
nm or greater, with the influence of the mixing of the layers above and
below the interfacial magnetic layer 2a being taken into consideration.
The magnetization directions of the interfacial magnetic layer 2a and the
interfacial magnetic layer 3a are parallel to each other.

[0114] Typical examples of TMR elements that are used with in-plane
magnetization are now described as stacked structures of Examples 8 to
12.

[0120] This example is another specific example structure of Example 6
shown in FIG. 8, and has the following stacked structure in which a 5-nm
thick CoFeTb film is used as the crystallization promoting layer 2b:

[0122] In this example, the magnetization directions of the CoFeB layer of
the interfacial magnetic layer 2a and the FePt layer of the magnetization
pinned film 2d can be made antiparallel to each other by adjusting the
CoFeTb composition. If the relative proportion of the rare earth element
Tb exceeds the compensation point, the magnetization directions of the
interfacial magnetic layer 2a and the magnetization pinned film 2d become
antiparallel to each other.

Example 12

[0123] This example is the same as Example 6 shown in FIG. 8, except that
the interfacial magnetic layer 2a is replaced with a synthetic structure
consisting of a first magnetic film, a nonmagnetic film, and a second
magnetic film. The structure of this example is as follows:

[0125] Next, TMR elements of Example 13 and Comparative Example formed by
a sputtering technique are described. For each of those TMR elements, the
area resistance RA and the TMR ratio were measured by an in-plane
energizing technique.

(Example 13

[0126] The TMR element of this example is a specific example of the TMR
element of Example 1 shown in FIG. 5, and has the following stacked
structure:

[0130] The results of the measurement show that the RA in Comparative
Example is approximately 20 kΩμm2, and the RA in Example 13
is approximately 10 kΩμm2. Meanwhile, the TMR ratio does
not become lower, and is maintained at a constant value.

[0131] A TMR element of Example 14 formed by the same sputtering technique
as above is now described.

Example 14

[0132] The TMR element of this example is the same as the TMR element of
Example 13, except that the 0.2-nm thick crystallization promoting layer
2b made of Ta is replaced with a 10-nm thick CoFeTb film. This TMR
element has the following stacked structure:

[0134] Through transmission electron microscopic (TEM) observation, the
interfacial magnetic layer 2a of Example 14 was observed. As a result of
cross-sectional TEM observation, crystallization in the CoFeB film of the
interfacial magnetic layer 2a was confirmed in Example 14. In Comparative
Example, however, the entire CoFeB film as the interfacial magnetic layer
2a was substantially an amorphous structure.

[0135] Further, the RA was measured by the in-plane energizing technique.
The results showed that the RA in Comparative Example was approximately
20 kΩμm-2, and the RA in Example having the CoFeTb film
inserted as the crystallization promoting layer 2b was lowered to 1
Ωμm-2.

[0136] As described above, in accordance with this embodiment, a
low-resistance TMR element can be obtained, and a magnetization reversal
of the magnetization free layer can be caused with a low current. Thus, a
low-resistance magnetoresistive element of a spin-injection write type
can be provided.

Second Embodiment

[0137] Next, a MRAM of a spin injection write type in accordance with a
second embodiment of the present invention is described.

[0138] The MRAM of this embodiment includes memory cells. FIG. 10 is a
cross-sectional view of one of the memory cells of the MRAM of this
embodiment. As shown in FIG. 10, the upper face of an MR element 1 is
connected to a bit line 32 via an upper electrode 31. The lower face of
the MR element 1 is connected to a drain region 37a of the source and
drain regions on the surface of a semiconductor substrate 36 via a lower
electrode 33, an extension electrode 34, and a plug 35. The drain region
37a, a source region 37b, a gate insulating film 38 formed on the
substrate 36, and a gate electrode 39 formed on the gate insulating film
38 constitute a selective transistor Tr. The selective transistor Tr and
the MR element 1 form the one memory cell of the MRAM. The source region
37b is connected to another bit line 42 via a plug 41. Alternatively, the
plug 35 may be provided under the lower electrode 33 without the
extension electrode 34, and the lower electrode 33 may be connected
directly to the plug 35. The bit lines 32 and 42, the electrodes 31 and
33, the extension electrode 34, and the plugs 35 and 41 are made of W,
Al, AlCu, Cu, and the likes.

[0139] In the MRAM of this embodiment, memory cells each having the same
structure as the memory cell shown in FIG. 10 are arranged in a matrix
form, so as to form the memory cell array of the MRAM. FIG. 11 is a
circuit diagram showing the principal components of the MRAM of this
embodiment. As shown in FIG. 11, memory cells 53 that are formed with MR
elements 1 and selective transistors Tr are arranged in a matrix form.
One end of each of the memory cells 53 arranged in the same column is
connected to the same bit line 32, and the other end is connected to the
same bit line 42. The gate electrodes (word lines) 39 of the memory cells
53 arranged in the same row are connected to one another, and are also
connected to a row decoder 51.

[0140] The bit line 32 is connected to a current source/sink circuit 55
via a switch circuit 54 such as a transistor. The bit line 42 is
connected to a current source/sink circuit 57 via a switch circuit 56
such as a transistor. The current source/sink circuits 55 and 57 supply
write current (inversion current) to the connected bit lines 32 and 42,
and remove the write current from the connected bit lines 32 and 42.

[0141] The bit line 42 is also connected to a read circuit 52. The read
circuit 52 may be connected to the bit line 32. The read circuit 52
includes a read current circuit, a sense amplifier, and the likes.

[0142] At the time of writing, the switch circuits 54 and 56 connected to
the memory cell on which writing is to be performed, and the selective
transistor Tr are turned on, so as to form a current path that runs
through the subject memory cell. One of the current source/sink circuits
55 and 57 functions as a current source, and the other one functions as a
current sink, in accordance with the information to be written.

[0143] As a result, the write current flows in the direction determined by
the information to be written. As for the write speed, it is possible to
perform spin-injection writing with a current having a pulse width of
several nanoseconds to several microseconds.

[0144] At the time of reading, a read current of such a small size as not
to cause a magnetization reversal is supplied to the subject MR element 1
by a read current circuit in the same manner as in the case of writing.
The read circuit 52 compares the current value or the voltage value
determined by the resistance value in accordance with the magnetization
state of the MR element 1, with a reference value. In this manner, the
read circuit 52 decides the resistive state.

[0145] At the time of reading, the current pulse width should preferably
be smaller than the current pulse width observed in a writing operation.
Accordingly, write errors with the current at the time of reading can be
reduced. This is based on the fact that the absolute value of the write
current is larger when the pulse width of the write current is smaller.

[0146] As described so far, in accordance with each embodiment of the
present invention, low-resistance TMR elements are used as memory
elements. Accordingly, the magnetization free layer can be caused to have
a magnetization reversal with a low current. Thus, a low-resistance
magnetoresistive random access memory of a spin-injection write type can
be provided.

[0147] Additional advantages and modifications will readily occur to those
skilled in the art. Therefore, the invention in its broader aspects is
not limited to the specific details and representative embodiments shown
and described herein. Accordingly, various modifications may be made
without departing from the spirit or scope of the general inventive
concepts as defined by the appended claims and their equivalents.